Copying of documents has been performed since the first recording of information in document form. Documents are produced using many procedures on many types of substrates incorporating many forms of information. Unauthorized copying of documents has also been occurring since the storage of information in document form first began. For much of the history of information documentation the procedures used to copy original documents have been sufficiently cumbersome and costly to provide a significant impediment to unauthorized copying, thus limiting unauthorized copying to original documents of high value (e.g. currency, etc.). However, in more recent times the introduction of new technologies for generating reproductions of original documents (e.g. electrophotography, etc.) has decreased the cost and inconvenience of copying documents, thus increasing the need for an effective method of inhibiting unauthorized copying of a broader range of restricted documents. The inability of convenient, low cost copying technologies to copy original documents containing color or continuous tone pictorial information restricted unauthorized copying primarily to black-and-white documents containing textual information and line art. Recently, the introduction of cost effective document scanning and digital methods of signal processing and document reproduction have extended the ability to produce low cost copies of original documents to documents containing color and high quality pictorial information. It is now possible to produce essentially indistinguishable copies of any type of document quickly, conveniently, and cost effectively. Accordingly, the problem of unauthorized copying of original documents has been extended from simple black-and-white text to color documents, documents containing pictorial images, and photographic images. In particular, restricting the unauthorized duplication of photographic images produced by professional photographers on digital copying devices has recently become of great interest.
U.S. Pat. No. 5,193,853 by Wicker, and U.S. Pat. No. 5,018,767 by Wicker, disclose methods for restricting the unauthorized copying of original documents on devices utilizing opto-electronic scanning by incorporating spatially regular lines into the document. The spacings of the lineations incorporated in the original document are carefully selected to produce Moire patterns of low spatial frequency in the reproduced document allowing it to be easily distinguished from the original and degrading the usefulness of the reproduction. Although the Moire patterns produced in the reproduced document are readily apparent to an observer, the required line pattern incorporated in the original document to produce the Moire pattern upon copying is also apparent to an observer. Additionally, production of the Moire pattern in the reproduced document requires specific scanning pitches be employed by the copying device. Accordingly, this method of restricting unauthorized document copying is applicable only to documents such as currency or identification cards where the required line pattern can be incorporated without decreasing the usefulness of the document. Application of this technique to high quality documents is unacceptable due to the degradation of quality and usefulness of the original document.
U.S. Pat. No. 5,444,779 by Daniele, discloses a method of restricting a document from unauthorized copying by the printing of a two-dimensional encoded symbol in the original document. Upon scanning of the original document in an initial step of a copying process, the encoded symbol is detected in the digital representation of the original document and the copying process is either inhibited or allowed following billing of associated royalty fees. U.S. patent application Ser. No. 08/593,772, filed Jan. 29, 1996, by Schildkraut et al., and entitled, "Copy Protection System," discloses the incorporation of a symbol of a defined shape and color into a document followed by detection of the symbol in a scanned representation of the document produced by the copying device. In both disclosures, the incorporated symbol is detectable by an observer and readily defeated by cropping the symbol from the original document prior to copying. In addition, incorporation of the symbol into the document is required in the generation of the original document leading to undesired inconvenience and additional cost. Accordingly, these methods of imparting restriction from unauthorized copying are unacceptable.
U.S. Pat. No. 5,390,003 by Yamaguchi, et al., U.S. Pat. No. 5,379,093 by Hashimoto, et al., and U.S. Pat. No. 5,231,663 by Earl, et al. disclose methods of recognizing a copy restricted document by the scanning and analysis of some portion of the original document and comparison of the signal obtained with the signals stored in the copying device. When the signal of a copy restricted document is recognized, the copying process is inhibited. This method of restricting from the unauthorized copying of documents is limited in application because the signals of all documents to be copy restricted must be stored in or accessible by each copying device of interest. Because the number of potential documents to be restricted is extremely large and always increasing, it is impractical to maintain an updated signature database in the copying devices of interest.
Methods of encrypting a digital signal into a document produced by digital means have been disclosed. These methods introduce a signal which can be detected in a copying system utilizing document scanning and signal processing. These methods offer the advantage of not being detectable by an observer, thus maintaining the usefulness of high quality restricted documents. However, implementation of these methods is dependent on digital production of original documents. Although increasing, production of high quality documents using digital means is still limited. Accordingly, this approach is not useful for restricting the unauthorized copying of high quality documents produced using non-digital production methods.
U.S. Pat. No. 5,412,718, by Narasimhalu, et al. discloses the use of a key associated with the physical properties of the document substrate which is required to decode the encrypted document. This method of restricting the unauthorized copying of documents is unacceptable for applications of interest to the present invention because it requires encryption of the original document, rendering it useless prior to decoding.
U.S. Pat. No. 5,752,152, filed Feb. 8, 1996, by John Gasper, et al., and entitled, "Copy Restrictive System" and U.S. patent application Ser. No. 08/598,785, also filed on Feb. 8, 1996, by John Gasper, et al., and entitled, "Copy Restrictive Documents" disclose pre-exposing color photographic paper to spots of blue light to produce an array of yellow microdots after chemical processing and a method of detecting these microdots in the end user's image during scanning performed by a digital printing device. Color photographic paper capable of forming yellow microdots after exposure to spots of blue light is of the color-negative type. The yellow microdots are most easily detected in areas of the image of low reflection density in all color records, usually referred to as the highlight areas, and for this reason they need to be exposed so as to form yellow microdots of low reflection density. If, however, their reflection density is made too low then the scanner of the digital copying device may be unable to detect them in typical scenes having a wide range of reflection densities. This sets tight tolerances on the acceptable range of microdot densities.
U.S. patent application Ser. No. 08/837,931, by John Gasper, et al. and entitled "Copy Restrictive System for Color-Reversal Documents" and U.S. Pat. No. 5,772,250, by John Gasper and entitled "Copy Restrictive Color-Reversal Documents," both filed on Apr. 10, 1997, disclose using color-reversal photographic media to create copy restrictive documents. Exposure of color-reversal photographic media to microdots of blue light prior to or after recording of the image exposure produces imperceptible (but scanner detectable) microdots after photographic processing. In areas of the scene of very low reflection density (highlight areas), however, there are no microdots present. It is therefore possible to form microdots in the recorded image that offer excellent detection by a digital copier in a region of reflection densities where they are not visually detectable. The advantages of improved scanner detectability and improved invisibility offered by employing color-reversal photographic media cannot be achieved in color-negative photographic media when the microdots are created by light exposure.